Conservation of momentum_elastic collision

The collision is elastic. You can use u1-u2 = v2-v1 only in paper 1 directly. However you may need to derive it if you are using it in a paper 2 or paper 3 question depending on the sequence of parts. This equation is not really required in the syllabus. Be cautious when using it . Do not use if the question ask for conservation of momentum or conservation of kinetic energy. This equation is the result of COM and COKe. Try moving it around and you have it.